A piston type liquid pump for a braking system of a vehicle is known in the art, for example, as disclosed in Japanese Patent Publication No. H07-035035. The liquid pump of the prior art has a pump chamber and a suction chamber, wherein the volume of the suction chamber is decreased when the volume of the pump chamber is decreased and the volume of the suction chamber is increased when the volume of the pump chamber is increased, in accordance with a reciprocal movement of a piston. The liquid pump pressurizes the brake fluid supplied from an inlet port to the pump chamber through the suction chamber, and discharges the pressurized brake fluid. As above, the suction of the brake fluid from the outside of the pump into the suction chamber and the discharge of the brake fluid to the outside of the pump are alternately carried out in accordance with the reciprocal movement of the piston.
When the liquid pump of the above type is applied to a braking system having a return fluid path, a pulsation is likely to be increased, because pressure variation is alternately carried out in the return fluid path as a result of the suction and discharge of the brake fluid. The above pulsation is generated not only in the return fluid path but also in a fluid circulation path through which the brake fluid is circulated. Then, it may cause a problem of noise generation, when the pulsation is increased in the return fluid path and/or the fluid circulation path.
According to another prior art, for example, as disclosed in Japanese Patent Publication No. H06-323240, a liquid pump of the piston type has a suction chamber, the volume of which is increased when the volume of a pump chamber is decreased, whereas the volume of the suction chamber is decreased when the volume of the pump chamber is increased. According to such prior art, the suction of the brake fluid from the outside of the pump into the suction chamber and the discharge of the brake fluid to the outside of the pump are concurrently carried out in accordance with the reciprocal movement of the piston. Pressure variation of the brake fluid in the return fluid path as well as the fluid circulation path is suppressed with such liquid pump, and the pulsation may be hardly caused.
It is desirable to make the suction volume of the pump closer to the discharge volume of the pump in order to suppress the pulsation.
According to the pump of the above prior art (JPH06-323240), the suction chamber is formed by an inner peripheral surface of a pump housing and an outer peripheral surface of the piston, wherein an inner diameter of the inner peripheral surface of the pump housing is made equal to or smaller than an inner diameter of a cylindrical space of the pump housing, into which one end of the piston is slidably inserted. In other words, an outer diameter of the piston on the opposite side to the cylindrical space is made smaller than an outer diameter of the piston on the side to the cylindrical space (at which the end of the piston is inserted into the cylindrical space). With such a structure, a necessary cross sectional area (when viewed in an axial direction of the cylindrical space) and thereby a necessary suction volume is achieved.
According to such a structure, however, the diameter of the other end of the piston is made smaller in order to increase the suction volume (to make the suction volume equal to or closer to the discharge volume). Such a small diameter portion of the piston is not always preferable in view of mechanical strength of the piston.